The present invention relates to a digital data communication system and, more particularly, to an adaptive channel impulse response estimation system for estimating an impulse response reliably at the receiving side of a channel which varies with time and suffers from distortions due to intersymbol intereference and an adaptive maximum likelihood sequence estimation (MLSE) system for demodulating received data correctly by reducing the influence of demodulation delay.
In digital communication over a channel which varies with time and suffers from intersmbol interference as mentioned above, the receiving side needs an adaptive equalizer adapting itself to the characteristic of the channel to demodulate received data correctly. If the impulse response of the channel is known, the adaptive equalizer can be constructed to satisfy a predetermined characteristic. It is therefore necessary to estimate the impulse response of the time-varying channel from a received signal, so that the equalizer may follow the channel. A conventional implementation for estimating a channel impulse response from a received signal consists in applying the output of a decision unit to a replica generating filter which generates a replica of a received signal and is implemented as a transversal filter, comparing the replica with the actual received signal, and controlling the tap coefficients of the filter in such a manner as to reduce a difference therebetween, as disclosed in, for example, Proakis "Digital Communications", McGraw-Hill, 1983 (referred to as PRIOR ART I hereinafter). In this kind of system, the tap coefficients of the replica generating filter are outputted as an estimated channel impulse response, and an adaptive equalizer is controlled in matching relation to the estimated channel impulse response. However, estimating a channel impulse response from a decision result is disadvanageous in that a decision error would cause the replica generating filter to diverge and thereby render the operation of the adaptive equalizer itself unstable, resulting in error propagation.
To reduce decision errors, the adaptive equation system may be implemented by MLSE using Viterbi algorithm, as taught in, for example, Hayes "The Viterbi Algorithm Applied to Digital Data Transmission", IEEE, Communication Society, 1975, No. 13 (hereinafter referred to as PRIOR ART II). This scheme successfully reduces decision errors and, therefore, stabilizes the operation of the replica generating filter so long as the channel varies slowly. However, when the channel variation is rapid, the ability of the filter to follow the channel variation is degraded due to demodulation delay particular to the MLSE scheme with the result that the decision error is aggravated.
Even linear equation or decision feedback equation cannot eliminate the above-discussed problems since the estimated channel impulse response achievable therewith is unstable.